1. Field of the Invention
The present invention relates to an image data processing apparatus which can process image data.
2. Description of the Related Art
There have been heretofore proposed several kinds of image input devices. As one of the image input devices, a scanner can be referred. The scanner is constructed to apply a light ray to an original, writing or printing, receive a reflected ray, and convert the reflected ray into an electric signal with a CCD (Charge Coupled Device)sensor for the purpose of inputting original image data to an image processing apparatus such as a computer.
The relation between the light ray input from the original to the CCD sensor and an output voltage of the CCD sensor has a characteristic that it changes linearly when the CCD sensor receives strong reflected ray (bright) but it does non-linearly when the CCD sensor receives weak light ray (gloom). That is, in the latter case, the output voltage of the CCD sensor depends on the change of light intensity appearing on the light-receptacle surface less precisely than in the former case. It means that a bright portion included on the original image can be crisply represented but a dark portion cannot be so represented.
This is because a shift register included in the CCD sensor has as high a transfer efficiency as 99.9999% but the transfer efficiency of a sensor section to the shift register is as low as about 95%.
To enhance the transfer efficiency of the sensor section to the shift register, as disclosed in the Japanese Patent Application Laying Open (KOKOKU) No. 52-4436, there has been proposed an improvement which is constructed to apply constant biased light ray to the CCD sensor independently of the scanning light ray and increase an absolute amount of charges stored in a potential well.
As another improved method for enhancing the transfer efficiency, as disclosed in the Japanese Patent Application Laying Open (KOKAI) No. 57-142068, there has been also proposed an improvement which is constructed to gate the output of the CCD sensor in synchronous to a reset pulse for extracting effective signal components, integrate the effective signal components, and hold the result as a sample.
As another improved method, as disclosed in the Japanese Patent Application Laying Open (KOKAI) No. 58-106948, there has been also proposed an improvement which is constructed to rapidly transfer residual charges while the CCD sensor is storing the charges and open a transfer gate as stopping a more rapid transfer clock signal after finishing output of a reading-start signal.
As an alternative method, as disclosed in the Japanese Patent Application Laying Open (KOKAI) No. 59-64963, there has been further proposed an improvement which is constructed to extend the light-emitting time when the quantity of light traveled from a light source is made lower and shorten the light-emitting time when it is made higher, for the purpose of constantly keeping the photoelectric output of the CCD sensor stable.
As a further alternative method, as disclosed in the Japanese Patent Application Laying Open (KOKAI) No. 59-221072, there has been further proposed an improvement which is constructed to vary a charge-storing time in a half-tone reading mode and fix it in a black-and-white binary signal reading mode in accordance with a control signal. The improvement results in enhancing the accuracy of a captured image.
The aforementioned improvements, however, cannot sufficiently overcome the foregoing shortcoming that the output characteristic of the CCD sensor makes it impossible to crisply read out a dark portion of the original.